This invention relates to a fluidized catalytic cracking (FCC) process and apparatus in which spent catalyst particles are sent to a regenerator before being sent back to a reaction zone while other catalyst particles are recycled back to the cracking zone without regeneration. Specifically, this invention relates to controlling the circulation rate of unregenerated, spent catalyst recycled back to the reaction zone.
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regenerator within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to generally by those skilled in the art as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams facilitates the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
One such configuration is disclosed in U.S. Pat. No. 5,451,313 B1 in which a portion of the spent catalyst is stripped and sent to the regeneration zone where it is regenerated and sent to a mixing vessel at the base of the riser and another portion of the spent catalyst is sent from the stripping vessel to the mixing vessel at the base of the riser without undergoing regeneration. The cooler spent catalyst mixes with the hotter regenerated catalyst to produce a lower overall temperature of the catalyst contacting the feed. This configuration reduces the localized overheating of the feed or the severity of the feed heating caused by the large temperature differentials between the feed and the catalyst which both contribute to feed overcracking.
In FCC systems, spent catalyst is typically sent from the base of the stripping vessel to the regenerator through a spent catalyst conduit. Regenerated catalyst is passed from the regenerator vessel through a regenerated catalyst conduit to the base of the reaction zone. The circulation rate of the spent catalyst through the spent catalyst conduit is maintained by a spent catalyst control valve, and the circulation rate of the catalyst through the regenerated catalyst conduit is maintained by a regenerated catalyst control valve. Spent catalyst that will not undergo regeneration is recycled either from the separator vessel or, specifically, the stripping vessel to the base of the riser by a recycled catalyst conduit. A recycled catalyst control valve regulates the recycled catalyst circulation rate. The respective catalyst circulation rate through the spent catalyst control valve, the regenerated catalyst control valve and the recycled catalyst control valve is approximated using the density in the respective catalyst conduit, the differential pressure across the control valve and the position of the control valve relative to fully opened.
The spent catalyst control valve automatically controls the circulation rate of spent catalyst from the separator vessel to the regenerator to maintain a constant catalyst level in the separator. The regenerated catalyst control valve to maintain a constant temperature of the separator vessel automatically controls the circulation rate of hot regenerated catalyst from the regenerator to the riser. The controls that operate each of the spent catalyst and regenerated catalyst control valves also include a low differential pressure override. If the differential pressure across either slide valve drops to a very low or negative value, the override will close the control valve. This minimizes the possibility of reverse flow in the conduits, either air entering the separator vessel or feed entering the regenerator, which are hazardous situations.
No direct measured variable has been used to control the recycled catalyst circulation rate through the recycled catalyst control valve. Therefore, an automatic control is needed, so the operator does not have to adjust the spent catalyst circulation rate based on a change in the feed rate or the regenerated catalyst circulation rate.
U.S. Pat. Nos. 2,743,998 B1, 3,591,783 B1, 3,964,876 B1, 4,220,622 B1 and 4,234,411 B1 all disclose FCC systems with automatic controls for regulating the FCC process. However, none of these references pertains to recycling spent catalyst to the reaction zone without undergoing regeneration or a means for controlling the circulation rate of such spent catalyst. Accordingly, it is an object of this invention to provide a means for automatically controlling the circulation rate of spent catalyst to the reaction zone without undergoing regeneration.
In one embodiment, the present invention relates to an apparatus for the fluidized catalytic cracking of hydrocarbons The apparatus comprises a reaction zone in which blended catalyst is contacted with a hydrocarbon feed. A separator vessel receives effluent from the reaction zone and separates the effluent into a vapor product and spent catalyst. A recycled catalyst conduit communicative with the separator vessel passes spent catalyst to the reaction zone. The recycled catalyst conduit includes a recycled catalyst control valve for regulating spent catalyst circulation and instrumentation for determining the circulation rate of spent catalyst through the recycled catalyst conduit. A regeneration zone removes carbon from the spent catalyst to provide regenerated catalyst. A spent catalyst conduit communicative between the separator vessel and the regeneration zone passes spent catalyst to the regeneration zone. A regenerated catalyst conduit communicative between the regeneration zone and the reaction zone passes regenerated catalyst from the regeneration zone to the reaction zone. The regenerated catalyst is blended with the spent catalyst in the reaction zone to provide the blended catalyst The regenerated catalyst conduit includes instrumentation for determining the circulation rate of regenerated catalyst through the regenerated catalyst conduit. A recycled catalyst controller sets a position of the recycled catalyst control valve dependent on whether a relationship between the circulation rate of spent catalyst through the recycled catalyst conduit and the circulation rate of regenerated catalyst through the regenerated catalyst conduit meets a preset condition.
In a further embodiment, the relationship between the circulation rate of spent catalyst through the recycled catalyst conduit and the circulation rate of regenerated catalyst through the regenerated catalyst conduit is the ratio of a sum of the circulation rate of spent catalyst through the recycled catalyst conduit and the circulation rate of regenerated catalyst through the regenerated catalyst conduit to the circulation rate of regenerated catalyst through the regenerated catalyst conduit.
In another embodiment, the present invention relates to an apparatus for the fluidized catalytic cracking of hydrocarbons that comprises a riser in which blended catalyst is contacted with a hydrocarbon feed. A separator vessel receives effluent from the riser and separates the effluent into a vapor product and spent catalyst. A recycled catalyst conduit communicative with the separator vessel passes spent catalyst to a base of the riser. The recycled catalyst conduit includes a recycled catalyst control valve for regulating spent catalyst circulation and instrumentation for determining the circulation rate of spent catalyst through the recycled catalyst conduit. A regenerator vessel removes hydrocarbons from the spent catalyst to provide regenerated catalyst. A spent catalyst conduit communicative between the separator vessel and the regenerator vessel passes spent catalyst to the regenerator vessel. A regenerated catalyst conduit communicative between the regenerator vessel and the base of the riser passes regenerated catalyst from the regenerator vessel to the base of the riser. The regenerated catalyst is blended with the spent catalyst to provide the blended catalyst in the riser. The regenerated catalyst conduit includes instrumentation for determining the circulation rate of regenerated catalyst through the regenerated catalyst conduit. A recycled catalyst controller sets a position of the recycled catalyst control valve dependent on whether a relationship between the circulation rate of spent catalyst through the recycled catalyst conduit and the circulation rate of regenerated catalyst through the regenerated catalyst conduit meets a preset condition.
In a further embodiment, the present invention relates to a process for the fluidized catalytic cracking of hydrocarbons. The process comprises contacting blended catalyst with a hydrocarbon feed in a reaction zone. The effluent is separated from the reaction zone into vapor product and spent catalyst. The spent catalyst is recycled to the reaction zone. A recycled catalyst control valve regulates circulation of the spent catalyst. A circulation rate of spent catalyst to the reaction zone is determined. The spent catalyst is passed from the reaction zone to a regeneration zone. Hydrocarbons are removed from the spent catalyst in the regeneration zone to provide regenerated catalyst. The regenerated catalyst is passed from the regeneration zone to the reaction zone, and a circulation rate of regenerated catalyst from the regeneration zone to the reaction zone is determined. The regenerated catalyst is blended with the spent catalyst in the reaction zone to provide the blended catalyst. An adjusted setting is signaled to the recycled catalyst control valve if a relationship between the circulation rate of spent catalyst to the reaction zone and the circulation rate of regenerated catalyst to the reaction zone does not meet a preset condition.
In a still further embodiment, the relationship between the circulation rate of spent catalyst and the circulation rate of regenerated catalyst is the ratio of a sum of the circulation rate of spent catalyst and the circulation rate of regenerated catalyst to the circulation rate of regenerated catalyst.
Additional objects, embodiments, and details of this invention will become apparent from the following detailed description.